This invention relates to screen plates and more particularly to plate-type screens which are used in pressure screening apparatus for removing contaminant particles from papermakers' stock.
High turbulence pressure screening apparatus of the kind shown, for example, in U.S. Pat. No. 4,155,841 issued May 22, 1979 to Chupka and Seifert, and assigned to the same assignee as this invention, is used in the preparation of papermaking stock. A suspension of liquid and paper fibers, which may in varying degrees contain undesirable rejects or contaminate particles, is supplied to the inlet of the apparatus as disclosed, for example, in the '841 patent, where it is applied to an annular cylindrically shaped screen having specifically designed slots or perforations therethrough. Typically, the paper stock is fed to the interior of a vertically oriented cylindrical screen, and the rejects are withdrawn from one end of such a screen, while the accepts pass through the slots or perforations in the screen and are collected at a location outwardly of the screen. Additionally, rotating foils or vanes are positioned either inside or outside the screen surface, in closely adjacent relation to the screen surface, to reduce the plugging of the screen slots or holes.
Such screens, as used in the separating apparatus, may further be provided with a plurality of generally axially-extending, radially spaced bars on the inlet surface of the screen which, in Patent '841, is radially inner surface of the screen. The bars cooperate with the rotating foils to reduce flow movement parallel to the surface of the screen and cause regions of turbulence in the stock adjacent the inlet side of the screen cylinder, to improve screening as disclosed for example in U.S. Pat. No. 4,410,424 issued Oct. 18, 1983 to Chupka and Seifert and assigned to the same assignee as this application.
In the design of such screen plates, it is desirable that the slots or holes present a sufficient percentage of open area as to provide adequate capacity. However, increasing the number of slots to increase the percent of open area, with conventionally formed slots, results in a substantial loss of metal of the plate and a corresponding decrease in the plate's strength. This is due to the fact that conventional metal cutting practices result in the formation of substantial relief areas on either side of the slot, often dependent upon the thickness of the plate itself. Commonly, such plates range between 0.25" and 0.50" thick, with 0.31" thickness being typical, with a size approximately 9" high and 24" in diameter being typical, although screen sizes may be as large as 60" high and 60" in diameter. Accordingly, the limitations of the machining or cutting step often define the extent of openness or percent of open area which can practically be achieved in a given plate.
In the manufacture of such screen plates, the turbulence-inducing bars have commonly been separately formed and welded in the desired position. In order to prevent warpage, this has commonly resulted in the use of bars of reasonably heavy stock, such as 0.25" stock material, although smaller bars, would normally be preferred to permit the foils to pass closer to the slots than is possible when using thicker or larger bars. The bars, when applied to a slotted plate, also tend to warp the lots and may have the effect of rendering the slot size so variable that the cylinder must be scrapped. Further, the act of welding on the land areas induces and causes weaknesses in the cylinders.
In conventional screen plates as described above, the material of the plate and material of the bar is subject to wear by friction and erosion over time. Such wear results in a loss of distinctiveness of the individual orifices or slots by reason of the rounding off of the lines of demarcation or edges between the inner or inlet surface of the plate and the orifices, as well as the edges of the bars, and result in an undesirable effective increase in orifice diameters or slot widths. There is accordingly a need for a manufacturing technique and process for making screen plates in which substantially harder materials may be economically used to extend the effective life of the plate, and to increase the percentage of open area without sacrificing strength.
There further exists a need to recondition or rebuild an existing screen plate or cylinder, either where it was not originally made with wear bars and to rebuild original wear bars which have become worn. Conventional screen plates are further characterized by openings, such as slots or holes, which are normal to the surface, that is, the openings or slots extend in the direction of the shortest distance between the inlet and outlet surfaces. Further, in the case of slots, such slots are commonly either circumferential or axial to the cylinder, and are not arranged in distinctive or unique patterns.